December 29, 2013 Subject:
It's a glitchy copy.
Every online copy of this episode suffers from a field sequence problem that wasn't caught when the original broadcast tapes were digitized for the DVDs. There's a few glitches throughout the episode but starting around the 16 minute mark it gets very bad.
Ripping it from a DVD is no good because the bad copy is on both the US and UK editions.
The only ways this will get fixed is to obtain the original films ($16,000 according to Tim Hunkin) or broadcast tapes, or for someone to still have an old VHS or Beta recording off an original broadcast.
The rest of the episodes are fine, no digitizing errors.